METHODS AND COST OF CONSTRUCTING ARCH AND GIRDER BRIDGES.
The construction problems in arch and girder bridges of moderate spans are simple, and with the exception of center construction and arrangement of plant for making and placing concrete, are best explained by citing specific examples of bridge work. This is the arrangement followed in this chapter.
CENTERS.—The construction of centers is no less important a task for concrete arches than for stone arches. This means that success in the construction of concrete arches depends quite as much upon the sufficiency of the center construction as it does upon any other portion of the work. The center must, in a word, remain as nearly as possible invariable in level and form from the time it is made ready for the concrete until the time it is removed from underneath the arch, and, when the time for removal comes, the construction must be such that that operation can be performed with ease and without shock or jar to the masonry. The problem of center construction is thus the two-fold one of building a structure which is immovable until movement is desired and then moves at will. Incidentally these requisites must be obtained with the least combined expenditure for materials, framing, erection and removal, and with the greatest salvage of useful material when the work is over. The factors to be taken count of are it, will be seen, numerous and may exist in innumerable combinations.
Fig. 148.—Center for 50 ft. Arch Span (Supported).
Fig. 149.—Center for 50-ft. Arch Span (Cocket).
Centers may be classified into two types: (1). Centers whose supports must be arranged so as to leave a clear opening under the center for passing craft or other purposes, and (2) centers whose supports can be arranged in any way that judgment and economy dictate. Centers of the first class are commonly called cocket centers. As examples of a cocket and of a supported center and also as examples of well thought out center design we give the two centers shown by Figs. 148 and 149, both designed for a 50-ft. span segmental arch by the same engineer. The development of the center shown by Fig. 148 into the cocket center shown by Fig. 149 is plainly traceable from the drawings. In respect to the center shown by Fig. 149 which was the construction actually adopted we are informed that 16,464 ft. B. M. were required for a center 36 ft. long, that the framing cost about $12 per M. ft. B. M., with carpenters' wages at $4 per day, and that the cost of bolts and nuts was about $1.50 per M. ft. B. M. With lumber at $20 per M. ft. B. M., this center framed and erected would cost about $35 per M. ft. B. M. As an example of framed centers for larger spans we show by Fig. 158 the centers for the Connecticut Avenue Bridge at Washington, D. C., with costs and quantities; other references to costs are contained in the index.
A center of very economical construction is shown by Fig. 159, and is described in detail in the accompanying text. The distinctive feature of this center is the use of lagging laid lengthwise of the arch and bent to curve. Another example of this form of construction may be found in a 3-span arch bridge built at Mechanicsville, N. Y., in 1903. The viaduct was 17 ft. wide over all, and consisted of two 100-ft. spans and one 50-ft. span. Pile bents were driven to bed rock, the piles being spaced 6 ft. apart and the bents 10 ft. apart. Each bent was capped with 10×12-in. timber. On these caps were laid four lines of 10×12-in. stringers, and 8×10-in. posts 3 ft. apart were erected on these stringers, and each set of four posts across the arch was capped with 8×10-in timbers the ends of which projected 3 ft. beyond the faces of the arch. The tops of these cross caps were beveled to receive the lagging which was put on parallel with the center line of the viaduct, sprung down and nailed to the caps. This lagging consisted of rough 1-in. boards for a lower course, on top of which was laid 1-in. boards dressed on the upper sides. Hardwood wedges were used under the posts for removing the centers. In the centers, forms and braces for the three arches there were used 140,000 ft. B. M. of lumber. The structure contained 2,500 cu. yds. of concrete.
Another type of center that merits consideration in many places is one developed by Mr. Daniel B. Luten and used by him in the construction of many arches of the Luten type of reinforced concrete arch. The particular feature of this type of arch is that in shallow streams for bridges of ordinary span the ends of the arch ring are tied together across stream by a slab of concrete reinforced to take tension. This slab is intended to serve the double purpose of a tie to keep the arch from spreading and thus reduce the weight of abutments and of a pavement preventing scour and its tendency to undermine the abutments. Incidentally this concrete slab, which is built first, serves as a footing for the supports carrying the arch center.
As an illustration of the center we choose a specific structure. In building a 95-ft. span, 11-ft. 1-in. rise arch bridge at Yorktown, Ind., in 1905, the centers were designed so as to avoid the use of sand boxes or wedges. Ribs of 2×12-in. pieces cut to the arc of the arch soffit were supported on uprights standing on the concrete stream bed pavement. The uprights were so proportioned by Gordon's formula for columns that without bracing they would be too light to support the load of concrete and earth filling that was to come upon them, but when braced at two points dividing the uprights approximately into thirds they would support their loading rigidly and without buckling. The design in detail was as follows: The uprights near the middle of the span were about 15 ft. long and were spaced 7 ft. apart across the stream and 3 ft. apart across the bridge. Each upright then was to support a loading of concrete of 7 ft.×3 ft.×26 ins. and an earth fill 1 ft.×7 ft.×3 ft., or a total load of about 9,000 lbs. Applying Gordon's formula for struts with free ends,
| f S | ||
| P | = | ——————————————————— |
| l² | ||
| I + ——————— | ||
| 125h² |
where P is the total load = 9,000 lbs., f is fibre stress for oak—1,600 lbs., l is length of strut in inches and h is least diameter of strut in inches, it was found that for a length of 15 ft. a 7×7-in. upright would be required to satisfy the formula, but for a length of 5 ft., which would result from bracing each strut at two points, a 4×4-in. timber satisfied the formula. Therefore, 4×4-in. timbers braced at two points were used for the longest uprights. About 30 days after the completion of the arch the bracing was removed from the uprights, beginning at the ends of the span and working towards the middle. As the bracing was being removed the uprights gradually yielded, buckling from 4 to 6 ins. from the vertical and allowing the arch to settle about ¼ in. at the crown. This type of center has been successfully employed in a large number of bridges.
Figure 150 shows a center for a 125-ft. span parabolic arch with the amount and character of the stresses indicated and with a diagram of the actual deflections as measured during the work.
Fig. 150.—Center for 125-ft. Span Parabolic Arch with Diagram of Deflections.
In calculating centers of moderate span there is seldom need of more than the simple formulas and tables given in Chapter IX. When the spans become larger, and particularly when they become very large—over 200 ft.—the problem of calculating centers becomes complex. None but an engineer familiar with statics and the strengths of materials and knowing the efficiency of structural details should be considered for such a task. Such computations are not within the intended scope of this book, and the design of large centers will be passed with the presentation of a single example, the center for the Walnut Lane Bridge at Philadelphia, Pa.
The main arch span of the Walnut Lane Bridge consists of twin arches spaced some 16 ft. apart at the crowns and connected across by the floor. Each of the twin arch rings has a span of 232 ft. and a rise of 70¼ ft., is 9½ ft. thick and 21½ ft. wide at the skewback and 5½ ft. thick and 18 ft. wide at the crown. The plan was to build a center complete for one arch ring and then to shift it along and re-use it for building the other arch ring. The centering used is shown in diagram by Fig. 151. It consists of five parts: (1) Six concrete piers running the full width of the bridge upon which the structure was moved; (2) a steel framework up to E G, called the "primary bent"; (3) a separate timber portion below the heavy lines E I and W' I'; (4) the "main staging" included in the trapezoid E I W' I', and (5) the "upper trestle" extending from I I' to the intrados.
Fig. 151.—Center for 232-ft. Span Arch at Philadelphia, Pa.
The primary bent consists of four I-beam post bents having channel chords, the whole braced together rigidly by angles. Each bent is carried on 1½ ft.×6 in. steel rollers running on a track of 19×½ in. plate on top of the concrete piers. Between the primary bents and the main staging, and also between the main staging and the upper trestles are lifting devices. The mode of operation planned is as follows: When the center has been erected as shown and the arch ring concreted the separate stagings under K I and K' I' are taken down. Next the portions under the lines I E and I' W' will be taken down and erected under the second arch. Finally the remainder of the center will be shifted sidewise on the rollers to position under the second arch.
MIXING AND TRANSPORTING CONCRETE.—The nature of the plant for mixing and handling the concrete in bridge work will vary not only with varying local conditions but with the size and length of the bridge. For single span structures of moderate size the concrete can be handled directly by derricks or on runways by carts and wheelbarrows. For bridges of several spans the accepted methods of transport are cableways, cars and cars and derricks. Typical examples of each type of plant are given in the following paragraphs, and also in the succeeding descriptions of the Connecticut Avenue Bridge at Washington, D. C., and of a five-span arch bridge.
Cableway Plants.—The bridge was 710 ft. long between abutments and 62 ft. wide; it had a center span of 110 ft., flanked on each side by a 100-ft., a 90-ft. and an 80-ft. span. The mixing plant was located at one end of the bridge and consisted of a Drake continuous mixer, discharging one-half at the mixer and one-half by belt conveyor to a point 50 ft. away, so as to supply the buckets of two parallel cableways. The mixer output per 10-hour day was 400 cu. yds. and the mixing plant was operated at a cost of $27 per day, making the cost of mixing alone 6¾ cts. per cu. yd. The sand and gravel were excavated from a pit 4½ miles away and delivered by electric cars to the bridge site at a cost of 50 cts. per cu. yd. Two 930-ft. span Lambert cableways set parallel with the bridge, one 25 ft. each side of the center axis, were used to deliver the concrete from mixer to forms. The cableway towers were 70 ft. high and the cables had a deflection of 35 ft.; they were designed for a load of 7 tons, but the average load carried was only 3 or 4 tons. These cableways handled practically all the materials used in the construction of the bridge. They delivered from mixer to the work 400 cu. yds. of concrete 450 ft. in 10 hours at a cost of 2 cts. per cu. yd. for operation.
Fig. 151a.—Cableway for Concreting Bridge Piers.
Another example of cableway arrangement for concreting bridge piers is shown by Fig. 151a. The river was about 800 ft. wide, about 3 ft. deep and had banks about 20 ft. high. The piers were about 21 ft. high. The towers for the cableway consisted of a 55-ft. derrick without boom, placed near the bank on the center line of the piers and well guyed and a two-leg bent placed in the middle of the river and held in place by four cable guys anchored to the river bottom. A ¾-in. steel hoisting cable was stretched from a deadman on shore, about 150 ft. back of the derrick, and followed along the center line of the piers, past the derrick just clearing it, to the bent in the middle of the river. At the top of this bent was a 16-in. cable block. Through this block the cable passed down and was made fast to a weight, consisting of a skip loaded with concrete until the cable had the required tension, and a pitch of 18 to 20 ft. from center of river to anchor on shore. In order to secure the required pitch from the shore to the river bent the boom fall of the derrick was hooked onto the cable at the foot of the mast, and then, by going ahead on the single drum hoisting engine, was raised to the mast head. This gave the cable a pitch of 18 to 20 ft. from mast head to top of bent in river. The carriage vised on the cableway consisted of two 16-in. cable sheaves with iron straps, forming a triangle, with a chain hanging from the bottom point, to which was attached the 5 cu. ft. capacity concrete bucket. The concrete was mixed on a platform at the foot of the mast. When ready for operation the chain on the carrier was hooked to the bucket of concrete, the engine started, and both bucket and cable raised, the former running by gravity to the pier. The speed of descent was governed by the height to which the cable was raised on the derrick, and as the bucket neared the dumping point the engine was slacked off and the cable leveled. The bucket was dumped by a man on a staging erected on the pier form. For the return of the bucket the engine was slacked off and the weight on the river bent would pull the cable tight so that the pitch would be toward the shore and the bucket could run down the grade to the mixing platform, the speed being governed as before by leveling the cable. When the piers were completed to the middle of the river the engine and derrick were taken over to opposite side of the river, the bent being left in the middle, and the work continued. By using the extreme grade of the cable it was found that the bucket would run from the platform to the bent (400 ft.) in less than 35 seconds.
Fig. 152.—Sketch Showing Car and Trestle Plant for Concreting an Arch Bridge.
Car Plant for 4-Span Arch Bridge.—The bridge had four 110-ft. skew spans, and a total length of 554 ft. The mixing plant was located alongside one abutment on a side hill so that sand and stone could be stored on the slope above. The mixer was set on a platform high enough to clear cars below. Above it and to the rear a charging platform reached back to the stone and sand piles. Side dump cars running on a track on the charging platform took sand and stone to the mixer and cement was got from a cement house at charging platform level. The concrete for the abutment adjacent to the mixer was handled in buckets by a guy derrick. A trestle, Fig. 152, was then built out from the mixer to the first pier; this trestle was so located as to clear the future bridge about 20 ft. and was carried out from shore parallel to the bridge until nearly opposite the pier site, where it was swung toward and across the pier. The concrete was received from the mixer in bottom dump push cars; these cars were run out over the pier site and dumped. When the first pier had been concreted to springing line level, the main trestle was extended to opposite the second pier and the branch track was removed from over the first pier and placed over the second pier. This operation was repeated for the third pier. The last extension of the main track was to the far shore abutment, where the bodies of the cars were hoisted by derrick and dumped into the abutment forms. The derrick was the same one used for the first abutment having been moved and set up during the construction of the intermediate piers. To construct the arches a second trestle was built composed partly of new work and partly of the staging for the arch centers. This trestle rose on an incline from the mixer to the first pier across which it was carried at approximately crown level of the arch. The concrete for the portion of the pier above springing line and for the lower portions of the haunches was dumped direct from the cars. For the upper parts of the arch the concrete was brought to the pier track in two-wheel carts on push cars and thence these carts were taken along the arch toward shore on runways. When the first arch had been concreted the second trestle was extended to pier two and the operation repeated to concrete the second arch.
Hoist and Car Plant for 21-Span Arch Viaduct.—The double track concrete viaduct replaced a single track steel viaduct, being built around and embedding the original steel structure which was maintained in service. The concrete viaduct consisted of 21 spans of 26 ft., 7 spans of 16 ft., and 2 spans of 22 ft. With piers it required about 15,000 cu. yds. of concrete. Two Ransome concrete hoists, one on each side of the original steel structure near one end, were supplied with concrete by a No. 4 Ransome mixer. The mixer discharged direct into the bucket of one hoist and by means of a shuttle car and chute into the bucket of the other hoist.
The shuttle car ran from the mixer up an incline laid with two tracks, one narrow gage and one wide gage, having the same center line. The car was open at the front end and its two rear wheels rode on the broad gage rails and its two forward wheels rode on the narrow gage rails. At the top of the incline the narrow gage rails pitched sharply below the grade of the broad gage rails so that the rear end of the car was tilted up enough to pour the concrete into a chute which led to the bucket of the hoist. The sand and gravel bins were elevated above the mixer and received their materials from cars which dumped directly from the steel viaduct.
The hoist buckets discharged into two hoppers mounted on platforms on the old viaduct. These platforms straddled two narrow gage tracks, one on each side of the old viaduct parallel to and clearing the main track. These side tracks were carried on the cantilever ends of long timbers laid across the old viaduct between ties. At street crossings the overhanging ends of the long timbers were strutted diagonally down to the outside shelf of the bottom chords of the plate girder spans. Six cars were used and the concrete was dumped by them directly into the forms; the fall from the track above being in some cases 40 ft. The hoists and shuttle car were operated by an 8½×12-in. Lambert derrick engine, the boiler of which also supplied steam to the mixer engine. The concrete cars were operated by cable haulage by two Lambert 7×10-in. engines.
The labor force employed in mixing and placing concrete, including form work, was 45 men, and this force placed on an average 200 cu. yds. of concrete per day. Assuming wages we get the following costs of different parts of the work for labor above:
| Item. | Per day. | Per cu. yd. |
| 1 timekeeper at $2.50 | $ 2.50 | $0.0125 |
| 1 general foreman at $5 | 5.00 | 0.0250 |
| 3 enginemen at $5 | 15.00 | 0.0750 |
| 1 carpenter foreman at $4 | 4.00 | 0.0200 |
| 12 carpenters at $3.50 | 42.00 | 0.2100 |
| 1 foreman at $4 | 4.00 | 0.0200 |
| 8 men mixing and transporting at $1.75 | 14.00 | 0.0700 |
| 13 men placing concrete at $1.75 | 22.75 | 0.1137 |
| 1 foreman finishing at $4 | 4.00 | 0.0200 |
| 4 laborers finishing at $1.75 | 7.00 | 0.0350 |
| ——— | ——— | |
| 45 men at $2.70 | $120.25 | $0.6012 |
It is probable that the carpenter work includes merely shifting and erecting forms and not the first cost of framing centers. No materials, of course, are included. It should be kept in mind that while the output and labor force are exact the wages are assumed.
Traveling Derrick Plant for 4-Span Arch Bridge.—The bridge consisted of four 70-ft. arch spans and was built close alongside an old bridge which it was ultimately to replace. The approach from the west was across a wide flat; at the east the ground rose more abruptly from the stream. Conditions prevented the use of a long spur track and also made it necessary to install all plant at and to handle all material from the west bank. A diagram sketch of the arrangement adopted is shown by Fig. 153.
Fig. 153.—Sketch Showing Traveling Derrick Plant for Concreting an Arch Bridge.
The track from the west approached the existing bridge on an embankment 25 ft. high. A spur track 175 ft. long from clear post to end was built on trestle as shown. The cement house and mixer platform were placed at the foot of the embankment at opposite ends of the spur track. Between the two the slope of the embankment was sheeted with 1-in. boards and a timber bulkhead 4 ft. high was built along the toe of the sheeting. Stone, sand and coal were stored behind the bulkhead on the sheeting. A runway close to the bulkhead connected the cement house with the mixer platform, all materials to the mixer being wheeled in barrows on this runway. A ¾-cu. yd. Smith mixer was set on a platform 5 ft. above ground with its discharge end toward the stream. Beginning under this platform a service track was carried across the flat and stream to the extreme end of the east abutment. This track consisted of three rails, two rails 4 ft. apart next to the work and a third rail 25 ft. from the first. The 4-ft. gage provided for cars carrying concrete buckets from the mixer and the 25-ft. gage provided for a traveling derrick; 18-lb. rails were used and they proved to be too light, 40-lb. rails are suggested. The derrick consisted of a triangular platform carrying a stiff leg derrick with a 25-ft. mast and mounted on five wheels. The wheels were double flange 16 ins. diameter and cost $30 each, being the most expensive part of the derrick. The derrick was made on the ground and took four carpenters between 3 and 4 days to build. Derrick and 350 ft. of service track, including pole trestle across the stream, cost between $600 and $800. The derrick was moved by means of a cable wrapped around one spool of the Flory double-drum hoisting engine and leading forward and back to deadmen set at opposite ends of the service track. Cars carrying concrete buckets were run out on the 4-ft. gage track and the buckets were hoisted by the derrick and dumped into a ½-cu. yd. car running on a movable transverse track across the bridge. This transverse track was necessary to handle the concrete to the far side of the work, the derrick being set too low and the boom being too short to reach. The derrick was used to handle material excavated from the pier foundations and also to tear down the centers and spandrel forms. Some rather general figures on the cost of this bridge are given by Mr. H. C. Harrison, the contractor. They are:
| Materials: | Total. |
| 6,000 bbls. cement at $2.05 | $12,300 |
| 2,500 cu. yds. sand at $0.80 | 2,000 |
| 5,000 cu. yds. stone at $0.85 | 4,250 |
| 260 M. ft. B. M. lumber at $17 | 4,420 |
| ——— | |
| Total | $22,970 |
| Labor: | |
| Cofferdams, excavation and pumping | $ 3,000 |
| Forms, falseworks and centers | 2,000 |
| Mixing and placing concrete | 4,000 |
| Placing reinforcement | 400 |
| Removing falseworks, forms, etc. | 1,200 |
| One coat pitch and paper | 150 |
| Building plant, etc. | 2,250 |
| ——— | |
| Total | $13,000 |
Mr. Harrison states that including plant cost, delays, floods and incidentals the cost per cubic yard of concrete was $8 and that excluding these items the cost was $6 per cu. yd.
COST OF CONSTRUCTING CONCRETE HIGHWAY BRIDGE, GREENE COUNTY, IOWA.—The following is the itemized cost of constructing a reinforced concrete slab highway bridge, one of several built by the Highway Commissioners of Greene County, Iowa, in 1906. The figures are given by Messrs. Henry Haag and D. E. Donovan, the last being the foreman of the concrete gang doing the work. All bridges consist of 10 to 12-in. slabs reinforced with old steel rails and of abutments and wing walls reinforced with old rods, bars or angles selected from junk. This junk metal cost 0.6 cts. per pound and the rails cut to length cost 1.15 cts. per pound f. o. b. cars. The work was done by a special gang, the men receiving $1.50 per day and board. As a rule the footings were made 2 ft. wide and as high as need be to get above the water and dirt. Before the footing concrete set steel rods, bars or angles were placed; they were long enough to reach the height of the wall and 3 to 6 ins. into the slab. The forms for the abutment and wing walls and for the floor slab were then erected complete before any more concrete was placed. No carpenter was employed, every man on the job having been taught to take his certain place in the work, then, the forms being erected, every man had his particular place in the work of mixing and placing the concrete. The foreman saw that the reinforcement was properly placed and watched over the accuracy of the work generally. The concrete was allowed to set on the centers for from 30 to 40 days; the other form work was taken down after three days and travel over the bridge permitted after three or four days. The concrete was mixed wet. The bridge whose cost is given was 22 ft. wide and 16 ft. span with 2-ft. wing walls.
The foundations are 4 ft. deep and 2½ ft. wide. The walls on top of the foundations are 7 ft. high, 18 ins. wide at the base, and battered up to 14 ins. at the top for wings and 12 ins. at top for walls. The floor is 22 ft. by 18 ft. and 1 ft. thick. The wheel guard is 12 ins. thick by 14 ins. wide and 32 ft. long. The itemized cost of this bridge, containing 73 cu. yds. of concrete, is as follows:
| Materials. | Total. | Per cu. yd. |
| 70 cu. yds. gravel at 70 cts | $ 49.00 | $0.6726 |
| 10 cu. yds. broken stone at 70 cts | 7.00 | 0.0959 |
| 75 bbls. cement at $2.20 | 165.00 | 2.2603 |
| 7,000 lbs. steel rails at 1.15 cts | 80.50 | 1.1027 |
| 1,000 lbs. junk rails at 0.6 cts | 6.00 | 0.0819 |
| 200 ft. B. M. lumber wasted at $29 | 5.80 | 0.0794 |
| 15 lbs. nails at 3 cts | 0.45 | 0.0061 |
| Labor and Supplies: | ||
| 2 days excavation at $14 | 28.00 | 0.3835 |
| ¾ day foundation at $14 | 10.00 | 0.1369 |
| 1½ days building forms at $14 | 21.00 | 0.2876 |
| 2 days filling forms at $14 | 28.00 | 0.3835 |
| Hauling lumber and tools | 8.00 | 0.1096 |
| Hauling cement and tools | 18.00 | 0.2465 |
| Taking off forms | 2.30 | 0.0315 |
| 1,000 lbs. coal at $4 per ton | 2.00 | 0.0274 |
| ——— | ——— | |
| Total cost | $431.05 | $5.9054 |
In round figures the cost per cubic yard of concrete in the finished bridge was $5.90. Summarizing we have the following cost per cubic yard of concrete in place:
| Item. | Per cu. yd. |
| Cement | $2.26 |
| Steel | 1.22 |
| Lumber | 0.22 |
| Gravel and stone | 0.76 |
| Labor | 1.41 |
| Coal | 0.03 |
| —— | |
| Total | $5.90 |
The average cost of concrete in place for all the work done in Greene County by day labor was $6.25 per cu. yd. In the job itemized above the bank caved in, causing an extra expense for removing the earth. The gravel used in this bridge was very good clean river gravel.
METHOD AND COST OF CONSTRUCTING TWO HIGHWAY GIRDER BRIDGES.—The following account of the methods and costs of constructing two slab and beam highway bridge decks on old masonry abutments is taken from records kept by Mr. Daniel J. Hauer. The first bridge was a single span 15 ft. long that replaced wooden stringers and floor that had become unsafe; the second was two short spans of a steel bridge that was too light for the traffic of the road, and it was torn down and moved elsewhere, by the county authorities. The work was done by contract, and in each case consisted of building the reinforced floor and girders on the old masonry walls that were in good condition. While the work was going on traffic was turned off the bridges, fords being used instead. Figure 154 shows a sketch of the cross-section of the floor and girders. In Example I the girders had a depth below the floor of 12 ins. and were of the same width. In Example II the girders were 14 ins. wide and had a depth below the floor of 18 ins. The floors on both bridges were 6 ins. thick. Kahn bars were used for reinforcement.
Fig. 154.—Cross-Section of Concrete Girder Bridge.
Example I.—This bridge was but little more than 5 ft. above the stream, which was shallow and not over 7 ft. wide, unless swollen by floods. The bottom for several hundred feet on either side of the bridge was covered with coarse sand and gravel, that had pebbles in it from the size of a goose egg down. This was taken from the stream by men with picks and shovels and hauled to the site of the work with wheelbarrows, and then screened so as to separate the gravel from the sand. As it was found that the sand was so coarse that it would take more cement than the specifications called for in a 1-2½-5 mixture, some much finer sand was bought and mixed with it. For the privilege of taking the sand from the stream $1 was paid the property owner. This was done to get a receipt and release from him, rather than as an attempt to pay royalty on the gravel and sand. This dollar is included in the cost of the labor in getting these materials.
The cost of materials per cubic yard for the bridge was as given below, the mixture being as stated above. The cement cost $1.40 per barrel, delivered at the bridge.
| Per Cu. Yd. | |
| Steel | $2.50 |
| Gravel and sand | .75 |
| Sand (bought) | .30 |
| Cement | 1.57 |
| —— | |
| Per cubic yard | $5.12 |
It is of interest to note the cost of the gravel and sand, as this includes the cost of digging it, wheeling it in a wheelbarrow an average distance of 100 ft., and then screening it and putting it in two stock piles. The proportion of bought sand used with the creek sand was one-half.
The old wooden floor and stringers had to be torn down. This was done at a cost of $1.30 per M. ft. B. M., and furnished 60 per cent. of the lumber needed for forms. The floor boards were 3-in. yellow pine planks, and the stringers 6×12-in. timbers, rather heavy, but money was saved by using them. The 6×12-in. timbers were used for props for the centering. Additional lumber was bought, delivered at the site of the bridge, for $20.84 per M. ft. B. M.
In framing and erecting the forms the carpenter had laborers helping him, he doing only carpenter's work, the laborers carrying and lifting all pieces wherever possible. The carpenter's work was about 40 per cent. of the total labor cost, which was as follows per cubic yard of concrete:
| Tearing down old bridge | $0.08 |
| Lumber | .85 |
| Nails | .15 |
| Labor, carpenter | .77 |
| Labor, laborers | .96 |
| —— | |
| $2.81 |
The forms were torn down by laborers, with the assistance of a man and his helper, who were given the boards for this labor and to haul them away. This reduced this item somewhat, as it only amounted to 20 cts. per cu. yd.
The cost of the forms per thousand feet board measure was:
| New lumber | $20.82 |
| Nails | 1.44 |
| Labor, carpenter | 7.60 |
| Labor, laborers | 9.50 |
| Tearing down | 2.00 |
| ——— | |
| $41.36 |
All the men, including the carpenter, worked 10 hours per day, and were paid at the following rates:
| Carpenter | $2.50 |
| Sub-foreman | 2.00 |
| Laborers | 1.50 |
A regular foreman was not employed, but an intelligent and handy workman was given 50 cts. additional to lead the men and look after them when the contractor was not present.
A gang of six men did the work of mixing and placing, and as the stock piles were close by the mixing board no extra men were needed to handle materials. Water was secured from the stream in buckets for mixing. The mixture was made very wet. The cost per cubic yard for the entire structure was as follows:
| Preparing for mixing | $0.04 |
| Cleaning out forms | .06 |
| Handling steel | .03 |
| Mixing and placing | 1.15 |
| Ramming | .23 |
| —— | |
| $1.51 |
The cost of the contractor's expense of bidding, car fare, etc., is listed under general expense, and gives a total cost per cubic yard of:
| Materials | $ 5.12 |
| Erecting forms | 2.81 |
| Tearing down forms | .20 |
| Labor | 1.51 |
| General expense | 2.00 |
| ——— | |
| $11.64 |
Example II.—For this bridge both the stone and sand had to be bought. The bridge floor was nearly 14 ft. above the bottom of the stream, which was shallow. The wages paid were as follows for a 10-hour day:
| Foreman | $3.00 |
| Laborers | 1.50 |
Carpenters were paid $3 for an 8-hour day and time and a half for all overtime, which they frequently made.
For the girders a 1-2-4 mixture was used. The cement, delivered at the bridge, cost $1.21 per barrel, there being 8 cts. a barrel storage and 8 cts. a barrel for hauling included in this. The sand was paid for at an agreed price per cartload delivered, which averaged $1.34 per cu. yd. The stone was crushed so as to pass a 1½-in. ring in all directions. It was delivered at the bridge for $2.75 per cu. yd. This makes the cost per cubic yard for materials as follows:
| Steel | $1.41 |
| Cement | 2.18 |
| Sand | .67 |
| Stone | 2.75 |
| —— | |
| $7.01 |
For the floor a 1-3-5 mixture was used, making a cost for material of:
| Steel | $1.02 |
| Cement | 1.69 |
| Sand | .67 |
| Stone | 2.75 |
| —— | |
| $6.13 |
Two-inch rough pine boards were used to make the troughs for the girders, while 1-in. rough boards were used for the floors. These were all supported by 3×4-in. pine scantlings. This lumber cost delivered $17.50 per M. ft. B. M. Carpenters did all the framing, and erected it with the help of laborers. All the carrying of the lumber was done by laborers. This reduced the cost of the work, as the laborers' wages amounted to one-third of the whole cost. As soon as the forms were all in place, which was before the mixing of concrete commenced, the carpenters were discharged. The cost per cubic yard for forms was:
| Lumber | $2.82 |
| Nails | .05 |
| Labor, carpenters | 1.24 |
| Laborers | .62 |
| —— | |
| $4.73 |
The tearing down of the forms was done entirely by laborers at a cost of 61 cts. per cu. yd.
On concrete work it is also advisable to keep the cost of forms per thousand feet board measure, so as to have such data for estimating on new work. The cost per M. ft. on this job was:
| Lumber | $17.50 |
| Nails | .30 |
| Labor, carpenters | 7.65 |
| Laborers | 3.85 |
| Tearing down | 3.80 |
| ——— | |
| $33.10 |
The concrete was mixed by hand, water being carried in buckets from the creek. Ten to twelve men were worked in the gang under a foreman, and the concrete was wheeled from the mixing board to the forms in wheelbarrows. The mixture was made wet enough to run. The cost per cubic yard for the girders in detail was as follows:
| Foreman | $0.41 |
| Preparing for mixing | 0.14 |
| Cleaning out forms | 0.07 |
| Handling materials | 0.30 |
| Handling and placing steel | 0.40 |
| Mixing and placing | 0.87 |
| Ramming | 0.45 |
| —— | |
| $2.64 |
The cost of labor for the floor was:
| Foreman | $0.28 |
| Preparing for mixing | 0.08 |
| Cleaning out forms | 0.05 |
| Handling materials | 0.14 |
| Handling and placing steel | 0.08 |
| Mixing and placing | 0.87 |
| Ramming | 0.36 |
| —— | |
| $1.86 |
This gives a total cost per cubic yard for the concrete in the girders in the completed bridge as follows:
| Materials | $ 7.01 |
| Erecting forms | 4.73 |
| Tearing down forms | 0.61 |
| Labor | 2.57 |
| General expense | 1.60 |
| ——— | |
| $16.52 |
The cost per cubic yard for the floor was:
| Materials | $ 6.13 |
| Erecting forms | 4.73 |
| Tearing down forms | 0.61 |
| Labor | 1.86 |
| General expense | 1.60 |
| ——— | |
| $14.93 |
Included with this is an item for general expense, being expenses of the contractor in bidding on the work, car fare, and other items of expense in looking after the contract.
It will be noticed that a record is here given of three different mixtures and that the labor cost of mixing and placing increases with the richness of the mixture. This is because it takes a greater number of batches to the cubic yard. Record has also been given of cost of preparing the mixing board and other work necessary to start and clean up each day; also when stock piles could not be arranged close to the mixing board, of the cost of handling the materials. These items, it will be noticed, are large enough to be considered in estimating on new work. The cost of sweeping and cleaning out the forms has also been listed, as this work is extremely important.
The cost of the reinforcing steel is given in with the materials, but the labor of handling it and placing it in the forms is listed under labor. This naturally varies with the amount of steel needed, and with the Kahn bar it will vary from 10 cts. to 75 cts. per cubic yard, as the prongs of the bar must be bent into proper position and at times straightened, when bent in shipment. This cost seems large, but it is done with the ordinary labor, while with round rods a large amount of blacksmith work has to be done and a smith and his helper frequently must place them. The patent bars are all lettered and numbered as structural steel is, and can be placed under the direction of the foreman.
One striking lesson can be learned from the forming. It will be noticed that the cost for common labor for handling and helping to erect the forms was much larger in Example I than in Example II, although the bridge was higher in the latter instance. This was caused by the heavy timber that was used, and equaled an extra cost nearly 50 per cent. of the price of new lumber. It certainly speaks volumes against the use of unnecessarily heavy timber for concrete forms.
In bridge work the height of the floor above the stream to some extent governs the cost of the forms. This is made so by the extra lumber needed as props or falsework to support the forming, and also by the fact that men at some height above the ground do not work as quickly or as readily as they do nearer the ground. For high and long spans a derrick is sometimes needed for the work of placing the centering.
On these jobs the concrete was made so wet that with the proper tamping and cutting of the concrete in the forms the surfaces were so smooth that no plastering was needed.
MOLDING SLABS FOR GIRDER BRIDGES.—The bridges carry railway tracks across intersecting streets; the slabs rest on two abutments and three rows of columns so that there are two 24¼-ft. spans over the street roadway and one 10¾-ft. span over each sidewalk. The larger slabs were 24 ft. 3 ins. long, 33 ins. thick and 7 ft. wide; each contained 16¾ cu. yds. of concrete and weighed 36¾ tons. The smaller slabs were 10 ft. 9 ins. long, 17 ins. thick and 7 ft. wide; each contained 3.65 cu. yds. of concrete and weighed 7.8 tons. The weights were found by actual weighing. They make the weight of the reinforced slab between 160 and 162 lbs. per cu. ft. The concrete was generally 1 part cement and 4 parts pit gravel. The reinforcement consisted of corrugated bars. The method of molding was as follows:
Fig. 155.—Arrangement of Tracks and Forms for Molding Slabs for Girder Bridge.
Fig. 156.—Form for Molding Slabs for Girder Bridge.
A cinder fill yard was leveled off and tamped, then the forms were set up on both sides of two lines of railway track arranged as shown by Fig. 155. The exact construction of the forms for one of the larger slabs is shown by Fig. 156. The side and end pieces were so arranged as to be easily taken down and erected for repeated use. About 100 floors were used and they had to be leveled up each time used as the lifting of the hardened slab disarranged them. The side and end pieces were removed in about a week or ten days, but the slabs stood on the floor 90 days, being wetted each day for two weeks after molding.
The plant for mixing and handling the concrete was mounted on cars. A flat car had a rotary drum mixer mounted on a platform at its forward end. Beneath the mixer was a hopper provided with a deflector which directed the concrete to right or left as desired. Under the hopper were the ends of two inclined chutes extending out sidewise beyond the car—one to the right and one to the left—and over the slab molds on each side. Above the mixer was another platform containing a charging hopper, and from the rear of this platform an incline ran down to the rear end of the car and then down to the track rails. A car loaded with cement and gravel in the proper proportions was hauled up the incline by cable operated by the mixer engine, until it came over the topmost hopper into which it was dumped. This hopper directed the charge into the mixer below; the mixer discharged its batch into the hopper beneath from which it flowed right or left as desired into one of the chutes and thence into the mold. The chutes reached nearly the full length of the molds and discharged as desired over the ends into the far end of the mold or through a trap over the end of the mold nearest the car.
To the rear of the mixer car came a cement car provided with a platform overhanging its forward end. Two hoppers were set in this platform each holding a charge for one batch. Coupled behind the cement cars came three or four gravel cars. These were gondola cars and plank runways were laid along their top outer edges making a continuous runway for wheelbarrows on each side from rear of train to front of cement car. The sand and gravel were wheeled to the two measuring hoppers and the cement was handed up from the car below and added, the charge was then discharged into the dump car below and the car was hauled up the incline to the mixer as already described. Two measuring hoppers were used so that one was being filled while the other was emptied, thus making the work continuous.
The molding gang consisted of 33 laborers, two foremen and one engineman. This gang averaged 7 of the large slabs per 10-hour day and at times made as many as 9 slabs. When molding small slabs an average of 12 were made per day. This record includes all delays, moving train, switching gravel cars on and off, building runways, etc. The distribution of the men was about as follows:
| Handling Materials: | No. Men. |
| Shoveling gravel into wheelbarrows | 9 |
| Wheeling gravel to measuring hoppers | 9 |
| Emptying cement into measuring hoppers | 2 |
| Handling cement to men emptying | 1 |
| In charge of loading dump car | 1 |
| On top of cement car | 1 |
| Sub-foreman in charge | 1 |
| Mixing and Placing: | |
| Engineer | 1 |
| In charge of mixer | 1 |
| Hoeing and spreading in mold | 2 |
| Spading in mold | 2 |
| Finishing sides of block | 2 |
| General laborers | 3 |
| Foreman in charge | 1 |
| — | |
| Total men | 36 |
This gang mixed and placed concrete for 7 blocks or 117¼ cu. yds. of concrete per day. Assuming an average wage of $2 per day the cost of labor mixing and placing was 61.4 cts. per cu. yd. or $10.28 per slab. It is stated that the slabs cost $11.80 per cu. yd. on storage pile. This includes labor and materials (concrete and steel); molds; loading into cars with locomotive crane, hauling cars to storage yard and unloading with crane into storage piles, and inspection, incidentals, etc. To load the slabs into cars from storage piles, transport them to the work and place them in position is stated to have cost $2 per cu. yd. The slabs were placed by means of a locomotive crane being swung from the flat cars directly into place.
Fig. 157.—Sections Showing Construction of Connecticut Ave. Bridge.
METHOD AND COST OF CONSTRUCTING CONNECTICUT AVE. BRIDGE, WASHINGTON, D. C.—The Connecticut Ave. Bridge at Washington, D. C., consists of nine 150-ft. spans and two 82-ft. spans, one at each end, all full centered arches of mass concrete trimmed with tool-dressed concrete blocks. Figure 157 is a part sectional plan and elevation of the bridge, showing both the main and spandrel arch construction. This bridge is one of the largest concrete arch bridges in the world, being 1,341 ft. long and 52 ft. wide, and containing 80,000 cu. yds. of concrete. Its total cost was $850,000 or $638.85 per lin. ft., or $10.63 per cu. yd. of masonry. It was built by contract, with Mr. W. J. Douglas as engineer in charge of construction. The account of the methods and cost of construction given here has been prepared from information obtained from Mr. Douglas and by personal visits to the work during construction.
General Arrangement of the Plant.—The quarry from which the crushed stone for concrete was obtained was located in the side of the gorge at a point about 400 ft. from the bridge. Incidentally, it may be added, the fact that the contractor had an option on this quarry gave him an advantage of some $30,000 over the other bidders. The stone from the quarry was hoisted about 50 ft. by derricks and deposited in cars which traveled on an incline to a Gates gyratory crusher, into which they dumped automatically. The stone from the crusher dropped into a 600-cu. yd. bin under the bottom of which was a tunnel large enough for a dump car and provided with top gates by which the stone above could be dropped into the cars. The cars were hauled by cable to the mixer storage bin and there discharged. Sand was brought in by wagons and dumped onto a platform about 50 ft. higher than the bottom of the main stone bin. A tunnel exactly similar to that under the stone bin was carried under the sand storage platform. The sand car was hauled from this tunnel by cable to the mixer storage bin using the same cable as was used for the stone cars, the cable being shifted by hand as was desired. Cement was delivered to the mixer platform from the crest of the bluff by means of a bag chute.
The mixer used was one of the Hains gravity type. It had four drops and was provided with four mixing hoppers at the top. The concrete was made quite wet. The proportions of sand and water were varied to suit the stone according to its wetness and the percentage of dust carried by it. The head mixer regulated the proportions and his work was checked by the government inspector. From the bottom hopper the mixed concrete dropped into a skip mounted on a car.
Fig. 158.—Center for Connecticut Ave. Bridge (Elevation).
To distribute the skip cars along the work a trestle was built close alongside the bridge and at about springing line level. This trestle had a down grade of about 2 per cent. from the mixer. Derricks mounted along the centering and on the block molding platform lifted the skips from the cars and deposited them where the concrete was wanted. The skip cars were large enough for three skips but only two were carried so that the derricks could save time by depositing an empty skip in the vacant space and take a loaded skip away with one full swing of the boom. Altogether nine derricks were used in the bridge, four having 70-ft. booms and five having 90-ft. booms. These derricks were jacked up as the work progressed.
Fig. 158.—Center for Connecticut Ave. Bridge (Details).
Forms and Centers.—The forms for wall and pier work consisted of 1-in. lagging held in place by studs about 2 ft. on centers and they in turn supported by wales which were connected through the walls by bolts, the outer portions of which were removed when the forms were taken down.
The centers for the five 150-ft. arches were all erected at one time; those for the 82-ft. arches were erected separately. The seven centers required 1,500,000 ft. B. M. of lumber or 1,404 ft. B. M. per lineal foot of bridge between abutments, or 1,640 ft. B. M. per lineal foot of arch span. The centers for the main arch spans are shown in detail by Fig. 158; this drawing shows the sizes of all members and the maximum stresses to which they were subjected from the loading indicated, that is the arch ring concrete. The centers as a rule rested on pile foundations. Four piles to each post were used for the intermediate posts and two piles for the posts in the two rows next the piers. Concrete foundations, however, were put in Rock Creek and on the line of Woodley Lane Bridge where it was impracticable to drive piles. As considerable difficulty was experienced in driving the piles, the ground consisting mostly of rotten rock, it is thought that it would have cost less if the contractor had used concrete footings throughout.
Some of the costs of form work and centering are given. The cost of lumber delivered at the bridge site was about as follows:
| M. ft. B. M. | |
| Rough Virginia pine | $25 |
| Dressed Virginia pine lagging | 23 |
| Rough Georgia, sizes up to 12×12 ins. | 33 |
| Rough Georgia, sizes over 12×12 ins. | 35 |
| Rough oak lumber | 35 |
The following wages were paid: Foreman carpenter, $3.50; carpenters, $2 to $3; laborers, $1.70, with a few at $1.50. An 8-hour day was worked.
The cost, of formwork is given in summary as follows:
| Lagging per M. ft. (used twice): | |
| Lumber at $23 | $11.50 |
| Erection | 15.00 |
| ——— | |
| Total cost erected | $26.50 |
| Studding and rough boards used in place of lagging per M. ft. (used twice): | |
| Lumber at $25 | $12.50 |
| Erection | 10.00 |
| ——— | |
| Total cost erected | $22.50 |
| Wales per M. ft. (used six times): | |
| Lumber at $36 | $ 6.00 |
| Erection | 10.00 |
| ——— | |
| Total cost erected | $16.00 |
The total cost of the main arch span centers to the District of Columbia was $54,000 or $59 per lineal foot of arch span, or $37.33 per M. ft. B. M. The cost of center erection and demolition was as follows:
| Erection below springing line per M. ft. | $15 |
| Erection above springing line per M. ft. | 25 |
| Demolition | 5 |
The salvage on the centers amounted to $11 per M. ft. B. M.
The spandrel arch centers were each used twice and cost per M. ft. B. M. for
| Lumber at $25 per M. ft. | $12.50 |
| Erecting at $25 per M. ft. | 25.00 |
| Moving at $5 per M. ft. | 5.00 |
| Total per M ft. | 42.50 |
Molding Concrete Blocks.—The bridge is trimmed throughout with molded concrete blocks, comprising belt courses, quoin stones, chain stones, ring stones, brackets and dentils. The blocks were made of a 1-2-4½ concrete faced with a 1-3 mixture of Dragon Portland cement and bluestone screenings from ⅜-in. size to dust. They were cast in wooden molds with collapsible sides held together by iron rods. Each mold was provided with six bottoms so that the molded block could be left standing on the bottom to harden while the side pieces were being used for molding another block. The molding was done on a perfectly level and tight floor on mud sills, the perfect level of the molding platform having been found to be an important factor in securing a uniform casting. The blocks were molded with the principal showing face down and the secondary showing faces vertical. The facing mortar was placed first and then the concrete backing. Care was taken to tamp the concrete so as to force the concrete stone into but not through the facing. Mr. Douglas remarks that the back of the block should always be at the top in molding since the laitance or slime always flushes to the surface making a weak skin which will develop hair cracks. In this work the backs of the blocks were mortised by embedding wooden cubes in the wet concrete and removing them when the concrete had set. These mortises bonded the blocks with the mass concrete backing. The blocks were left to harden for at least 30 days and preferably for 60 days and were then bush hammered on the showing faces, some of the work being done by hand and some with pneumatic tools.
Some precautions necessary in the molding and handling of large concrete blocks were discovered in this work and merit mention. In designing blocks for molding it is necessary to avoid thin flanges or the flanges will crack and break off; blocks molded with a 2¼ in. flange projecting 1¾ ins. gave such trouble from cracking on this work that a flange 5 ins. thick was substituted. Provide for the method of handling the block so that dog or lewis holes will not come in the showing faces. Dog holes can be made with a pick when the concrete is three or four weeks old. When it is not practicable to use dogs, two-pin lewises can be used. The lewis holes should be cast in the block and should be of larger size than for granite; they should not be located too near the mortar faces. In turning blocks it is necessary to provide some sort of cushion for them to turn on or broken arrises will result. When the work will permit, it is desirable to round the arrises to about a ⅜-in. radius.
The following general figures of the cost of block work are available. Foreman cutters were paid $5 per day; foreman concrete workers $3 per day; stonecutters $4 per day; concrete laborers $1.70 per day, and common laborers $1.50 to $1.70 per day. Plain and ornamental blocks cost about the same, the large size of the ornamental blocks bringing down the cost. The following is given as the average cost of block work per cubic yard:
| Cement | $ 1.95 |
| Sand | 0.35 |
| Stone | 1.14 |
| Forms, lumber and making | 0.80 |
| Mixing and placing concrete | 1.50 |
| Dressing | 4.73 |
| Handling and setting | 2.00 |
| Superintendence, plant, incidentals at 25 per cent. | 3.12 |
| Condemnation at 5 per cent. | 0.78 |
| ——— | |
| Total cost blocks in place | $16.37 |
It will be seen that the largest single item in the above summary of costs is the item of dressing. This was done, as stated above, partly by hand and partly by pneumatic tools. Hand tooling cost about twice as much as machine tooling, but its appearance was generally better. The average cost of tooling the several forms of blocks is shown by Table XIX. For 42,190 sq. ft. the average cost was 26 cts. per sq. ft. or $2.34 per sq. yd., or $4.73 per cu. yd. of block work. This tooling was done by stone cutters, and was unusually high in cost.
Mass Concrete Work.—All parts of the bridge except the molded block trim were built of concrete deposited in place. Briefly, the molded blocks were set first and then backed up with the mass concrete deposited in forms and on centers. The only features of this work that call for particular description are those in connection with the main arch ring and the spandrel arch construction.
The main arch rings were concreted in transverse sections; Fig. 158 shows the size and order of construction of these sections. Back forms were necessary up to an angle of 45° from the spring line after which the concrete was made somewhat drier and back forms were not used. After Sections 1, 2, 3 and 4 had been concreted they were allowed to set and then the struts and back forms were taken out and the intervening sections were concreted. The large Sections 6 and 7 were concreted in five sections each, in order to permit the taking out of the timber struts supporting the sections above. The concrete in all sections was placed in horizontal layers as a rule and it is the judgment of the engineers in charge of this work that this is the preferable method.
Table XIX.—Showing Cost of Tooling Concrete Ornamental Blocks for Connecticut Avenue Bridge.
| Description. 1: 2: 4½ Concrete Backing. 1: 3 (Mortar Face). | Per Cubic Foot. | Per Superficial Foot of Showing Face. | ||||||
| Total Number Stones Cut. | Number cubic feet in each. | Total cubic feet cut. | Cost per cubic foot. | Superficial feet in each. | Total superficial feet. | Cost per superficial foot. | Number Super. ft. to one cubic foot. | |
| Brackets under Lamps and Rail Posts (Cap and Base) | 344 | 16.0 | 5,500 | $0.27 | 10.5 | 3,630 | $0.41 | 0.66 |
| Moulding under coping | 770 | 5.9 | 4,560 | 0.30 | 3.8 | 2,930 | 0.47 | 0.64 |
| Dentils between Moulding | 520 | 5.5 | 2,860 | 0.20 | 8.0 | 4,160 | 0.14 | 1.45 |
| Coping | 494 | 61.2 | 30,220 | 0.12 | 35.4 | 17,490 | 0.21 | 0.58 |
| Pedestal (3 courses) | 162 | 27.2 | 4,400 | 0.15 | 14.1 | 2,290 | 0.29 | 0.52 |
| Rail Posts (Top and Base) | 296 | 7.1 | 2,100 | 0.50 | 17.3 | 5,100 | 0.21 | 2.43 |
| Lamp Posts and Parapets over Piers (Top and Base) | 248 | 22.9 | 5,690 | 0.17 | 26.5 | 6,580 | 0.15 | 1.16 |
| Average of above—Totals | 2,834 | 19.5 | 55,330 | $0.17 | 14.8 | 43,190 | $0.26 | 0.77 |
Table XX.—Showing Cost of Mass Concrete Work per Cubic Yard.
[Transcriber's note: Table split]
| Description. | Cost Delivered on Mixer. | |||||
| Proportions. | Average Yardage for Days Run. | Cement. | Sand. | Stone. | Total Materials. | |
| Class A, in Piers | 1:2:4½ | 150 | 1.65 | 0.39 | 1.08 | 3.12 |
| Class A, in Arches | 1:2:4½ | 200 | 1.65 | 0.39 | 1.08 | 3.11 |
| Class B, in Piers —Solid Work | 1:3:6 | 160 | 1.40 | 0.42 | 1.23 | 3.05 |
| Class B, in Piers —Hollow Work | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 |
| Class B, in Spandrel Walls | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 |
| Class B, in Spandrel Arches | 1:3:6 | 200 | 1.40 | 0.42 | 1.23 | 3.05 |
| Class B, in Abutments | 1:3:6 | 150 | 1.40 | 0.42 | 1.23 | 3.05 |
| Class C, Filling over Bridge | 1:3:10 | 145 | 0.90 | 0.31 | 1.30 | 2.51 |
| Description. | Cost of Mixing and Placing. | ||
| Mixing. | Placing | Total Mixing and Placing | |
| Class A, in Piers | 0.09 | 0.21 | 0.30 |
| Class A, in Arches | 0.05 | 0.28 | 0.33 |
| Class B, in Piers —Solid Work | 0.09 | 0.18 | 0.27 |
| Class B, in Piers —Hollow Work | 0.11 | 0.36 | 0.47 |
| Class B, in Spandrel Walls | 0.11 | 0.40 | 0.51 |
| Class B, in Spandrel Arches | 0.07 | 0.26 | 0.33 |
| Class B, in Abutments | 0.11 | 0.24 | 0.35 |
| Class C, Filling over Bridge | 0.11 | 0.28 | 0.39 |
| Description. | Cost of Form Work. | ||||
| Erecting. | Taking Down | Lumber. | Total Form Work | Total Cost per Cubic Yard.[G] | |
| Class A, in Piers | 0.17 | 0.05 | 0.16 | 0.38 | $3.80 |
| Class A, in Arches | 0.08 | 0.03 | 0.10 | 0.21 | 3.66 |
| Class B, in Piers —Solid Work | 0.17 | 0.05 | 0.16 | 0.38 | 3.70 |
| Class B, in Piers —Hollow Work | 0.77 | 0.25 | 0.64 | 1.66 | 5.18 |
| Class B, in Spandrel Walls | 0.85 | 0.28 | 0.73 | 1.86 | 5.42 |
| Class B, in Spandrel Arches | 0.94 | 0.30 | 0.86 | 2.10 | 5.48 |
| Class B, in Abutments | 0.10 | 0.03 | 0.12 | 0.25 | 3.65 |
| Class C, Filling over Bridge | 0.00 | 0.00 | 0.00 | .... | 2.90 |
[G] Add 25% to the cost here tabulated for superintendence, plant and incidentals.
Considerable difficulty was experienced in building the large arches with a concrete block facing on account of the fact that the edges of the blocks are liable to chip off when any concentrated pressure is brought on them. In order to permit the ring of blocks to deform as the centering settled under its load, sheet lead was placed in the joints between blocks at the points corresponding with the construction joints between sections of the mass concrete backing. The deflection of the centers at the crown was a maximum of 3¼ ins. and a minimum of 2½ ins.
Table XXI—Detail Cost of Engineering and Inspection for Different Classes of Work.
| Engineering. | Inspection. | |||
| Kind of Work. | Total. | Unit. | Total. | Unit. |
| Class A, concrete, 23,500 cu. yds | $3,055.00 | $0.13 | $1,762.50 | $0.075 |
| Class B, concrete, 36,580 cu. yds | 3,658.00 | 0.10 | 1,646.10 | 0.045 |
| Class C, concrete, 2,150 cu. yds | 107.50 | 0.05 | 53.75 | 0.025 |
| Class D, concrete, 6,250 cu. yds | 1,875.00 | 0.30 | 4,687.50 | 0.75 |
| 1,000 M. ft. B. M. centering | 1,000.00 | 1.00 | 440.00 | 0.44 |
| Cement, 73,000 barrels | 365.00 | 0.005 | 730.00 | 0.01 |
| Earth filling, 50,000 cu. yds | 1,000.00 | 0.02 | 500.00 | 0.01 |
The centering of the main arches was not struck until the spandrel arches and all the work above the main arches to the bottom of the coping had been completed. The first and third spandrel arch on each side of the piers was made with an expansion joint in the crown. To permit further of the adjustment of the portion of the masonry above the backs of the main arches, the crown of the middle arch of each set of spandrel arches was left unconcreted until the center of the main arches had been struck. It may be noted here that the expansion joints in the first and third arches were carried up through the dentils and coping, and observations show that these joints are about ⅛ in. larger in winter than in summer.
The cost of the mass concrete work is shown in Table XX. These figures are based on the wages already quoted and the following: Foreman riggers, $4.50; riggers, $1.50 to $1.75 and $2; skilled laborers, $2; engineers, $3.50. The detail cost of engineering and inspection is shown in Table XXI.
ARCH BRIDGES, ELKHART, IND.—At the new Elkhart, Ind., yards of the Lake Shore & Michigan Southern Ry. the tracks are carried over a city street by concrete arches 40, 60 and 160 ft. long. These arches all have a span of 30 ft., a height of 13 ft. and a ring thickness at crown of 28 ins. The reinforcement consists of arch and transverse bars; the arch bars are spaced 6 ins. on centers 2½ ins. from both extrados and intrados, and the transverse bars are spaced 24 ins. on centers inside both lines of arch bars. The proportions of the concrete were generally 1 cement, 3 gravel and 6 stone. The gravel was a material dug from the foundations and was about 50 per cent. sand and 50 per cent. gravel, ranging up to the size of pigeons' eggs. The concrete was machine mixed and was mixed very wet.
The work was done by the railway company's forces, and Mr. Samuel Rockwell, Assistant Chief Engineer, gives the following figures of cost:
| Total. | Per cu. yd. | |
| Temporary buildings, trestles, etc. | $ 752.33 | $0.15 |
| Machinery, pipe fittings, etc. | 416.34 | 0.08 |
| Sheet piling and boxing | 1,006.12 | 0.21 |
| Excavation and pumping | 1,619.74 | 0.33 |
| Arch centers and boxing | 3,528.92 | 0.73 |
| ———— | —— | |
| Total | $7,323.45 | $1.50 |
| Concrete masonry: | ||
| Cement | 8,860.55 | 1.84 |
| Stone | 1,788.50 | 0.36 |
| Sand | 240.00 | 0.05 |
| Drain tile | 103.03 | 0.02 |
| Labor | 8,091.41 | 1.68 |
| ————— | —— | |
| Total concrete | $19,083.49 | $3.95 |
| Steel reinforcing rods | $ 3,028.39 | $0.63 |
| Engineering, watching, etc. | 508.40 | 0.11 |
| ————— | —— | |
| Grand total (4,833 cu. yds. concrete) | $29,943.73 | $6.19 |
ARCH BRIDGE, PLAINWELL, MICH.—The following figures of cost of a reinforced concrete arch bridge are given by Mr. P. A. Courtright. The bridge crosses the Kalamazoo River at Plainwell, Mich., and is 446 ft. long over all with seven arches of 54 ft. span and 8 ft. rise. The arch rings were reinforced with 4-in., 6-lb. channels bent to a radius of 70 ft. and spaced 1.9 ft. c. to c. The contract price of the bridge was $19,900.
The concrete was made of Portland cement and a natural mixture of sand and gravel in the proportions of 1-8 for the foundations, 1-6 for arches and spandrel walls and 1-4 for the parapet wall. The proportions were determined by measure; the wagon boxes being built to hold a cubic yard of sand and gravel. A sack of cement was taken as 1 cu. ft. For foundations the pit mixture was used without screening; stones over 4 ins. in diameter being thrown out at the pit or on the mixing board. For the arches and spandrel walls the gravel was passed over a 2-in. mesh screen on the wagon box. The aggregate for the parapet walls was screened to 1 in. largest diameter. The concrete was mixed in a McKelvey continuous mixer which turned the material eight times. The mode of procedure was as follows: The gravel was loaded upon wagons in the pit and hauled to a platform at the intake of the mixer. Half of the cement required in the concrete was then spread over the top of the load in the wagon box and the whole was dumped through the bottom of the wagon box onto the platform and spread with shovels. The remainder of the cement was spread over the mixture and the whole was shoveled by one man to a second man who shoveled it into the mixer. Water was added after the mixture had passed about one-third of the way through the mixer. The mixer delivered the concrete directly into wheelbarrows, by which it was delivered to the work. The concrete was spread in layers from 2 to 4 ins. in thickness and thoroughly rammed with iron tampers; two men were employed tamping for each man shoveling. The arches were concreted in three longitudinal sections, each section constituting a day's work. The work was done in 1903 and the concrete cost for mixing and placing:
METHODS AND COST OF CONSTRUCTING A FIVE-SPAN ARCH BRIDGE.—This bridge consisted of five elliptical arch spans of 40, 45, 60, 87 and 44 ft., carried on concrete piers. The arch rings were 12 ins. thick at the crowns and 18 ins. thick 5 ft. from the centers of piers and carried 4-in. spandrel walls; there were 1,000 cu. yds. of concrete in the arches and 600 cu. yds. in the piers. Each arch ring was reinforced by a grillage of longitudinal and transverse rods.
Fig. 159.—End View of Center for Short Elliptical Arch Spans.
Forms and Centers.—Figure 159 is an end view of the center arch. It consists of a series of bents, 6 ft. c. to c., the posts of each bent being 5 ft. c. to c. These posts are made of 2×6-in. Washington fir. Upon the heads of the posts rest 2×6-in. stringers, extending from bent to bent. Resting on these stringers are wooden blocks, or wedges, which support a series of cross-stringers, also of 2×6-in. stuff, spaced 2 ft. c. to c. On top of these cross-stringers rest the sheeting planks, which are 1×6-in. stuff, dressed on the upper side, and bent to the curve of the arch. This sheeting plank was not tongue and grooved, and a man standing under it, after it is nailed in place, could see daylight through the cracks. It looked as if it would leak like a sieve, and let much of the wet concrete mortar flow through the cracks, but, as a matter of fact, scarcely any escapes. Figure 160 shows a front view of a bent, and indicates the manner of sway bracing it with 1×4-in. stuff. Figure 161 shows the outer forms for the parapet wall, or concrete hand railing, and it will be noted that the cross-stringers are allowed to project about 3 ft. so as to furnish a place to fasten the braces which hold the upright studs. The inner forms for the parapet wall are shown in dotted lines. They are not put in place until all the concrete arch is built. Then they are erected and held to the outer forms by wire, and are sway braced to wooden cleats nailed to the top surface of the concrete arch.
Fig. 160.—Front View of Center for Short Elliptical Arch Spans.
Fig. 161.—Form for Parapet Wall for Arch Bridge.
For the five spans the total amount of lumber in the centers was in round figures 28 M. ft., distributed about as follows:
| Item. | Ft. B. M. |
| 1×6-in. sheeting | 5,600 |
| 2×6-in. longitudinal stringers | 2,600 |
| 2×6-in. cross stringers | 2,600 |
| 2×6-in. posts | 4,000 |
| 3×8-in. sills | 1,500 |
| 1×4-in. braces | 3,000 |
| Outer forms for spandrel walls | 4,000 |
| Inner forms for spandrel walls | 4,000 |
| ——— | |
| Total | 27,300 |
The aggregate span length of the arches was 276 ft., so that a little less than 100 ft. B. M. of lumber was used for centering per lineal foot of span. The superintendent at $5 per day and five carpenters at $3.50 per day erected the five centers in 18 days at a cost of $400, or a trifle more than $14 per M. ft. B. M.; the cost of taking down the centers was $2 per M. ft. B. M., and the lumber for the centers cost $24 per M. ft. B. M. making a grand total of $40 per M. ft. B. M. for materials and labor. As there were 1,000 cu. yds. of concrete in the arches and spandrels, the cost of centers and forms was $1.12 per cu. yd. This form lumber was, however, after taking down, used again in erecting a reinforced concrete building. Assuming that the lumber was used only twice, the cost of centers and forms for these five arches was less than 80 cts. per cu. yd. of concrete.
Shaping and Placing Reinforcement.—The 60 and 87-ft. spans were reinforced with 32 1-½-in. round longitudinal rods held in place by ½-in. square transverse rods wired at the intersections; the reinforcement of the smaller spans was exactly the same except that 1-in. diameter rods were used. To bend the longitudinal rods to curve, planks were laid on the ground roughly to the curve of the arch; the exact curve was marked on these planks and large spikes were driven part way into the planks along this mark. The end of a rod was then fastened by spiking it against the first projecting spike head and three men taking hold of the opposite end and walking it around until the rod rested against all the spikes on the curve. It took three men two 8-hour days to bend 46,000 lbs. of rods. Their wages were $2.50 each per day, making the cost of bending 0.03 ct. per pound, or 60 cts. per ton. It took a man 5 mins. to wire a cross rod to a longitudinal rod. With wages at $2.50 per day the cost of shaping and placing the reinforcement per ton was as follows:
| Item. | Per ton. |
| Bending rods | $0.60 |
| Shearing rods to lengths | 0.40 |
| Carrying rods onto bridge | 0.40 |
| Placing and wiring rods | 2.35 |
| —— | |
| Total | $3.75 |
Including superintendence the labor cost was practically $4 per ton, or 0.2 cts. per lb. Altogether 66,000 lbs. of steel was used for reinforcing 1,000 cu. yds. of concrete, or 66 lbs. per cu. yd. The cost of steel delivered was 2 cts. per lb., and the cost of shaping and placing it 0.2 ct. per lb., a total of 2.2 cts. per lb. or 2.2 × 66 = $1.45 per cu. yd. of concrete.
Mixing and Placing Concrete.—A Ransome mixer holding a half-yard batch was used. The mixer was driven by an electric motor. The concrete for the piers was a mixture of 1 part Portland cement to 7 parts gravel; for the arches, the concrete was mixed 1 to 5. The gravel was piled near the mixer, a snatch team being used to assist the wagons in delivering the gravel into a pile as high as possible. Run planks supported on "horses" were laid horizontally from the mixer to the gravel, so that big wheelbarrow loads could be handled. The barrows were loaded with long-handled shovels, and the men worked with great vigor, as is shown by the fact that four men, shoveling and wheeling, delivered enough gravel to the mixer in 8 hrs. to make 100 cu. yds. of concrete. We have, therefore, estimated on a basis of six men instead of four. The mixer crew was organized as follows:
| Per day. | |
| 6 men shoveling and wheeling | $12 |
| 2 men handling cement | 4 |
| 1 man handling water | 2 |
| 1 man dumping concrete | 2 |
| 2 men handling dump cars | 4 |
| 2 men handling hoisting rope | 4 |
| 4 men spreading and ramming concrete | 8 |
| 1 engineman | 4 |
| 1 foreman | 5 |
| Fuel, estimated | 3 |
| — | |
| Total | $48 |
The output of this crew was 100 cu. yds. per day. The concrete was hauled from the mixer in two small dump cars, each having a capacity of 10 cu. ft. The average load in each car was ¼ cu. yd. Ordinary mine cars were used, of the kind which can be dumped forward, or on either side. The cars were hauled over tracks having a gage of 18 ins. The rails weighed 16 lbs. per yard, and were held by spikes ¼×2½ ins. Larger spikes would have split the cross-ties, which were 3×4 ins. Only one spike was driven to hold each rail to each tie, the spikes being on alternate sides of the rail in successive ties. No fish plates or splice bars were used to join the rails, which considerably simplifies the track laying.
Fig. 162.—Trestle for Service Track.
Two lines of track were laid over the bridge. The tracks were supported by light bents, the cross-tie forming the cap of each bent, as shown in Fig. 162. The bents were spaced 3 ft. apart. There were two posts to each bent, toe-nailed at the top of the tie, and at the bottom to the arch sheeting plank. Two men framed these crude bents and laid the two rails at the rate of 150 lin. ft. of track per day, at a cost of 4 cts. per lin. ft. of track. As stated, there were two tracks, one on each side of the bridge, but they converged as they neared the concrete mixer, so that a car coming from either track could run under the discharge chute of the mixer; Fig. 163 shows the arrangement of the tracks at the mixer. The part of each rail from A to B (6 ft. long) was free to move by bending at A, the rail being spiked rigidly to the tie at A, leaving its end at B free to move. To move the end B, so as to switch the cars, a home-made switch was improvised, as shown in Figs. 163 and 164.
Fig. 163.—Arrangement of Service Tracks at Mixer.
Fig. 164.—Improvised Switch for Service Cars, General Plan.
It will be remembered that this bridge was a series of five arches. There was a steep grade from the two ends of the bridge to the crown of the center arch. Hence the two railway tracks ascended on a steep grade from the mixer for about 175 ft., then they descended rapidly to the other end of the bridge. Hence to haul the concrete cars up the grade by using a wire cable, it was necessary to anchor a snatch block at the center of the bridge. This was done by erecting a short post, the top of which was about a foot above the top of the rails. The post stood near the track, and was guyed by means of wires, and braced by short inclined struts. To the top of the post was lashed the snatch block through which passed the wire rope. Fig. 165 shows this post, P. About 10 ft. from the post P, on the side toward the mixer, another post, Q, was erected, and a snatch block fastened to it. When the hoisting engine, which was set near the concrete mixer, began hauling the car along the track, a laborer would follow the car. Just before the car reached the post Q, he would unhook the hoisting rope from the front end of the car, then push the car past the post Q, and hook the hoisting rope to the rear of the car. The car would then proceed to descend in the direction T, being always under the control of the wire rope, except during the brief period when the car was passing the post Q. Each of the two cars was provided with its own hoisting rope, and one engineer, operating a double drum hoist, handled the cars. The hoist was belted to an 8 HP. gasoline engine, no electric motor being available for the purpose.
Fig. 165.—General Plan of Rope Haulage System.
Fig. 166.
Fig. 167. Details of Haulage Rope Guides.
Where hauling is done in this manner with wire ropes, it is necessary to support the ropes by rollers wherever they would rub against obstructions. A cheap roller can be made by taking a piece of 2-in. gas pipe about a foot long, and driving a wooden plug in each end of the gas pipe. Then bore a hole through the center of the wooden plugs and drive a 1-in. round rod through the holes, as shown in Fig. 166. The ends of this rod are shoved into holes bored into plank posts, which thus support the roller. Where the rope must be carried around a more or less sharp corner, it is necessary to provide two rollers, one horizontal and the other vertical, as shown in Fig. 167.
When conveying concrete to a point on the bridge about 300 ft. from the mixer, a dump car would make the round trip in 3 mins., about ¼ min. of its time being occupied in loading and another ¼ min. in dumping. One man always walked along with each car, and another man helped pull the wire rope back.
Including the cost of laying the track and installing the plant, the cost of mixing and placing the 1,600 cu. yds. of concrete was only 55 cts. per cu. yd., in spite of the high wages paid. However, the men were working for a contractor under a very good superintendent.
Summing up the cost of the concrete in the arches of this bridge, we have:
| Per cu. yd. | |
| 1.35 bbl. cement at $3 | $4.05 |
| 1 cu. yd. gravel at $1 | 1.00 |
| 66 lbs. of steel in place at 2.2 cts. | 1.45 |
| Centers in place (lumber used once) | 1.12 |
| Labor, mix and place concrete | 0.55 |
| —— | |
| Total | $8.17 |
The cost of the nails, wire, excavation and plant rental is not available, but could not be sufficient to add more than 10 cts. per cu. yd. under the conditions that existed in this case.
CONCRETE RIBBED ARCH BRIDGE AT GRAND RAPIDS, MICH.—The bridge consisted of seven parabolic arch ribs of 75 ft. clear span and 14 ft. rise. The five ribs under the 21-ft roadway were each 24 ins. thick, 50 ins. deep at skewbacks and 25 ins. deep at crown; the two ribs under the sidewalks were 12 ins. thick and of the same depth as the main ribs. Each rib carried columns which supported the deck slab. Columns and ribs were braced together across-bridge by struts and webs. All structural parts of the bridge were of concrete reinforced by corrugated bars. The abutments were hollow boxes with reinforced concrete shells tied in by buttresses and filled with earth. There were in the bridge including abutments 884 cu. yds. of concrete and 62,000 lbs. of reinforcing metal, or about 70 lbs. of reinforcing metal per cu. yd. of concrete. Of the 884 cu. yds. of concrete 594 cu. yds. were contained in the abutments and wing walls and 290 cu. yds. in the remainder of the structure. (Fig. 168.)
Fig. 168.—Details of Ribbed Arch Bridge.
Centers.—The center for the arch consisted of 4-pile bents spaced about 12 ft. apart in the line of the bridge. The piles were 12×12 in.×24 ft. yellow pine and they were braced together in both directions by 2×10-in. planks. Each bent carried a 3×12-in. plank cap. Maple folding wedges were set in these caps over each pile and on them rested 12×12-in. transverse timbers, one directly over each bent. These 12×12-in. transverse timbers carried the back pieces cut to the curve of the arch. The back pieces were 2×12-in. plank, two under each sidewalk rib and four under each main rib of the arch. The back pieces under each rib were X-braced together. The lagging was made continuous under the ribs but only occasional strips were carried across the spaces between ribs. This reduced the amount of lagging required but made working on the centers more difficult and resulted in loss of tools from dropping through the openings. Work on the centers and forms was tiresome owing both to the difficulty of moving around on the lagging and to the cramped positions in which the men labored. Carpenters were hard to keep for these reasons.
Concrete.—A 1-7 bank gravel concrete was used for the abutments and a 1-5 bank gravel concrete for the other parts of the bridge. The concrete was mixed in a cubical mixer operated by electric motor and located at one end of the bridge. The mixed concrete was taken to the forms in wheelbarrows. The mixture was of mushy consistency. No mortar facing was used, but the exposed surfaces were given a grout wash. In freezing weather the gravel and water were heated to a temperature of about 100° F.; when work was stopped at night it was covered with tarred felt, and was usually found steaming the next morning.
Cost of Work.—The cost data given here are based on figures furnished to us by Geo. J. Davis, Jr., who designed the bridge and kept the cost records. Mr. Davis states that the unit costs are high, because of the adverse conditions under which the work was performed. The work was done by day labor by the city, the men were all new to this class of work, the weather was cold and there was high water to interfere, and work was begun before plans for the bridge had been completed, so that the superintendent could not intelligently plan the work ahead. Cost keeping was begun only after the work was well under way. Many of the items of cost are incomplete in detail.
The following were the wages paid and the prices of the materials used:
| Materials and Supplies: | |
| No. 1 hemlock matched per M. ft. | $20 |
| No. 1 hemlock plank per M. ft. | 17 |
| No. 2 Norway pine flooring per M. ft. | 19 |
| No. 2 yellow pine flooring per M. ft. | 20 |
| 12×12-in.×16-ft. yellow pine per M. ft. | 29 |
| 12×12-in.×24-ft. yellow pine, piling per M. ft. | 27 |
| Maple wedges per pair | 50 cts. |
| ½-in. corrugated bars per lb. | 2.615 cts. |
| ¾-in. corrugated bars per lb. | 2.515 cts. |
| ⅞-in. corrugated bars per lb. | 2.515 cts. |
| Coal per ton | $4 |
| Electric power per kilowatt | 6 cts. |
| Medusa cement per bbl. | $1.75 |
| Aetna cement per bbl. | 1.05 |
| Bank gravel per cu. yd. | 0.85 |
| Sand per cu. yd. | 0.66 |
| Carpenters per day | $3 to 3.50 |
| Common labor per day | 1.75 |
The summarized cost of the whole work, with such detailed costs as the figures given permit of computation, was as follows:
| General Service: | Total. | Per cu. yd. |
| Engineering | $451 | $0.512 |
| Miscellaneous | 75 | 0.084 |
| Pumping: | Total 110 days. | |
| Coal at $4 per ton | $210 | |
| Machinery, tools and cartage | 283 | |
| Labor | 497 | |
| —— | ||
| Total | $990 |
This gives a cost of $9 per day for pumping.
In more detail the cost of the various items of concrete work was as follows for the whole structure, including abutments, wing walls and arch containing 884 cu. yds.:
| Form Construction: | Total. | Per cu. yd. |
| Lumber and cartage | $1,547 | $1.75 |
| Nails and bolts | 129 | 0.15 |
| Tools | 110 | 0.12 |
| Labor, erecting and removing | 1,526 | 1.72 |
| ——— | —— | |
| Total | $3,312 | $3.74 |
Concrete Construction.
| Materials: | ||
| Aetna cement at $1.05 | $1,218 | $1.37 |
| Medusa cement at $1.75 | 499 | 0.56 |
| Sand at 66 cts. per cu. yd. | 37 | 0.04 |
| Gravel at 85 cts. per cu. yd. | 915 | 1.04 |
| ——— | —— | |
| Total materials | $2,669 | $3.01 |
| Mixing: | ||
| Machinery and supplies | $ 549 | $0.62 |
| Power at 6 cts. per kw. | 52 | 0.06 |
| Tools | 22 | 0.02 |
| Labor | 737 | 0.83 |
| —— | —— | |
| Total mixing | $1,360 | $1.53 |
| Placing concrete | $ 609 | $0.69 |
| Tamping concrete | $ 481 | $0.54 |
| Heating Concrete: | ||
| Apparatus and cartage | $ 47 | $0.05 |
| Fuel | 96 | 0.11 |
| Labor | 270 | 0.31 |
| —— | —— | |
| Total heating | $ 413 | $0.47 |
| Grand total | $8,844 | $9.98 |
Considering the abutment and wing wall work, comprising 594 cu. yds., separately, the cost was as follows:
| Forms: | Per cu. yd. |
| Materials | $1.20 |
| Labor | 1.09 |
| —— | |
| Total | $2.29 |
| Concrete: | |
| Materials | $2.92 |
| Labor | 2.38 |
| —— | |
| Total | $5.30 |
| Heating water and gravel | $0.70 |
| Grand total | $8.29 |
Considering the arch span, comprising 290 cu. yds., separately, the cost was as follows:
| Forms: | Per cu. yd. |
| Materials | $3.70 |
| Labor | 3.03 |
| —— | |
| Total | $6.73 |
| Concrete: | |
| Materials | $3.22 |
| Labor | 3.57 |
| Total | $6.79 |
| Grand total | $13.52 |